An actuator of the kind specified is described in one form in the specification of French published patent application No. FR 2 610 261A, in which the actuating force reducing means is in the form of a helical spring and a toothed wheel or pinion, which meshes with the toothed sector member. Such an arrangement involves an increase in the length of the toothed sector plate, in order to allow this toothed wheel or pinion to come into engagement with it. In addition, the toothed wheel and the toothed sector plate have to be suitably treated, so as to be able to withstand the substantial forces exerted in this part of the actuator. This gives rise to manufacturing difficulties, and also involves the risk of deformation and the possibility of cracking of the components.